Jackpacks are pack pre-stressing units which are used to pre-stress timber or cementitous mats, elongate clusters or other packs that are used for supporting the hanging wall. Jackpacks are manufactured from two steel sheets cut in various profiles. The sheets are welded together to form an inflatable steel pillow. An integral valve unit allows the filler nozzle to be connected for installation and maintains the internal pressure of the unit. A pressure relief valve can also be included which allows for controlled pressure release after installation. This allows the Jackpack to ‘deflate’ in a controlled manner until the load is supported purely by the support unit which the Jackpack pre-stressed on installation.